The invention relates to a headup display, in particular for a motor vehicle, having an image forming unit for forming an item of image information to be displayed that can be projected as light beams by the image forming unit directly or via an optical projection system onto a partially reflecting pane, in particular onto a windshield pane.
In the case of such headup displays, it is known to form the image forming unit as a liquid crystal display which produces the image information to be displayed. By means of a light source arranged behind the liquid crystal display and trans illuminating the liquid crystal display the image information produced by the liquid crystal display is projected as light beams directly or via an optical projection system onto a partially reflecting region of a windshield pane in the visual range of a driver of a motor vehicle and reflected to the driver from there. This produces a virtual image, visible to the driver, at a spacing of a few meters approximately in the area of the beginning of the radiator hood of the motor vehicle which the driver can view, without a problem, simultaneously with the surround field without accommodating the eyes.
In order for such a virtual image to be effectively detectable in the case of as far as possible all ambient conditions, in particular high ambient brightness, and for alphanumeric characters to be effectively readable, its luminance must as far as possible be greater than 5000 cd/m3. On the basis of the light losses at mirrors and/or lenses and at the windshield pane, this involves a luminance of 20,000-30,000 cd/m3 on the surface of the liquid crystal display. If the liquid crystal display is a colored liquid crystal display, a luminance of 500,000 cd/m3 must be produced by the light source behind the liquid crystal display because of the low transmission of a colored liquid crystal display of 4-5%. In this case, the colored liquid crystal display has a contrast of less than 150, and this leads to a lit-up background of the liquid crystal display that has a disturbing effect.
The required high luminance of the backlighting of the liquid crystal display requires a light source of correspondingly large dimensions with a high energy consumption, the large developed heat of which must be dissipated by large heat sinks, fans or other heat dissipators.
Moreover, the design of the headup display is complicated and requires a large installation space.